1. Field of the Invention
This invention relates generally to magnetic heads in disk drives, and more particularly to magnetic write heads having write coil structures with relatively low electrical resistances to reduce thermal protrusion.
2. Description of the Related Art
A write head is typically combined with a magnetoresistive (MR) or giant magnetoresistive (GMR) read head to form a magnetic recording head, certain elements of which are exposed at an air bearing surface (ABS). The write head comprises first and second pole pieces connected at a back gap that is recessed from the ABS. The first and second pole pieces terminate at the ABS where they define first and second pole tips, respectively. An insulation stack, which comprises a plurality of insulation layers, is sandwiched between the first and second pole pieces, and a coil layer is embedded in the insulation stack. A processing circuit is connected to the coil layer for conducting write current through the coil layer which, in turn, induces magnetic write fields in the first and second pole pieces. A non-magnetic gap layer is sandwiched between the first and second pole tips. Write fields of the first and second pole tips at the ABS “fringe” across the gap layer. In a magnetic disk drive, a magnetic disk is rotated adjacent to, and a short distance (fly height) from, the ABS so that the write fields magnetize the disk along circular tracks. The written circular tracks then contain information in the form of magnetized segments with fields detectable by the MR or GMR read head.
An MR read head includes an MR sensor sandwiched between first and second non-magnetic gap layers, and located at the ABS. The first and second gap layers and the MR sensor are sandwiched between first and second shield layers. In a merged MR head, the second shield layer and the first pole piece are a common layer. The MR sensor detects magnetic fields from the circular tracks of the rotating disk by a change in resistance that corresponds to the strength of the fields. A sense current is conducted through the MR sensor, where changes in resistance cause voltage changes that are received by the processing circuitry as readback signals.
A GMR read head includes a GMR sensor which manifests the GMR effect. In the GMR sensor, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. GMR sensors using only two layers of ferromagnetic material (e.g., nickel-iron, cobalt, or nickel-iron-cobalt) separated by a layer of nonmagnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. A GMR head is typically associated with a design in which the second shield layer and first pole piece are not a common layer. These pieces are separated by a non-magnetic material, such as alumina, or a metal that can be deposited using physical vapor deposition, RF sputtering, or electroplating techniques, for example.
One or more heads may be employed in a magnetic disk drive for reading and writing information on circular tracks of a rotating disk. A magnetic recording head is mounted on a slider that is carried on a suspension. The suspension is mounted to an actuator which places the magnetic head to locations corresponding to desired tracks. As the disk rotates, an air layer (an “air bearing”) is generated between the rotating disk and an air bearing surface (ABS) of the slider. A force of the air bearing against the air bearing surface is opposed by an opposite loading force of the suspension, causing the magnetic head to be suspended a slight distance (i.e. its fly height) from the surface of the disk. Fly heights are typically around 5–20 nanometers (nm) in today's disk drives.
It is generally desirable to minimize the fly height of a magnetic head. If the fly height is too large, it could adversely affect the performance of the read and write head. Unfortunately, any protrusion of metal layers at the ABS will make these layers dangerously close to the disk, especially in disk drives with low fly heights. This could result in head-to-disk crashes or disk scratches.
“Temperature-induced protrusion” (T-PTR) refers generally to the phenomenon where magnetic head materials physically and outwardly protrude from the ABS at elevated temperatures due to the differences in the coefficients of thermal expansion of the various layers which form the head. “Write-induced protrusion” (W-PTR) refers to protrusion due to heating of the magnetic head during the writing process. There are two contributors to W-PTR: (1) Joule heating produced in the write head coils; and (2) yoke core losses. Joule heating and yoke core losses are both induced with AC write current. W-PTR is dominated by the temperature gradient in the head structure, with the highest temperature regions being near the write coils and the yoke, and substrate material at ambient temperature.
Accordingly, what is needed is an improved magnetic head that provides a reduced thermal protrusion so that head-to-disk crashes and/or disk scratches can be avoided.